Digital crt system for displaying a precessing waveform and its derivative

ABSTRACT

A system for displaying the derivative of a waveform together with the waveform. Successive digital samples of the waveform are circulated with each sample controlling a dot display during a respective vertical trace. As new samples are taken and stored in the circulating register which contains a number of stages greater than the number of vertical sweeps in each frame, the waveform appears to move from right to left across the screen. Because samples of the original waveform are taken at a fixed rate, samples of its derivative can be derived by subtracting successive samples from each other. The derivative samples are formed as the original samples are shifted out of the circulating register prior to the vertical sweeps.

1 DIGITAL CRT SYSTEMFOR DISPLAYING A PRECESSING WAVEFORM AND ITS DERIVATIVE [75] inventor: Christopher C, Day, Newtonville,

Mass.

[73] Assignee: American Optical Corporation,

Southbridge, Mass.

[22] Filed: Apr. 21, 1972 [21] Appl. No.: 246,221

[52] US. Cl 340/324, A, 235/171, 235/198 [51] Int. Cl. G06f 3/14 [58] Field of Search 340/324 A, 324 AD;

[56] References Cited UNITED STATES PATENTS 3,538,317 11/1970 Fukuda 235/150.5l 3,648,270 3/1972 Metz et a1.... 3,406,387 10/1968 Werme 3,686,662 8/1972 Blixt et al. 235/198 12] f Mn A l-WQRD CONVERTER REGlSTER B LA(-) LB(-) [111 3,768,093 [451 Oct.23, 1973 6/1969 Dertouzos et a1 340/324 A 2,745,985

5/1956 Lewis 235/171 [57] ABSTRACT A system for displaying the derivative of a waveform together with the waveform. Successive digital samples of the waveform are circulated with each sample controlling a dot display during a respective vertical trace. As new samples are taken and stored in the circulating register which contains a number of stages greater than the number of vertical sweeps in each frame, the waveform appears to move from right to left across the screen. Because samples of the original waveform are taken at a fixed rate, samples of its derivative can be derived by subtracting successive samples from each other. The derivative samples are formed as the original samples are shifted out of the circulating register prior to the vertical sweeps.

20 Claims, 5 Drawing Figures 1- SHOT TRGRM 7 VRT RTRCEl') DISPLAY UNIT HRZ RTRCE (-1 -TRGR, SET

L0 HRZ RTRCE This invention relates to waveform display systems, and more particularly to systems which provide a display of both a waveform and its derivative.

A moving or precessing display (typically developed on the face of a c hode reay tube) provides the same visual effect $a waveform on a paper trace which is moved behind a window. For example, if the waveform moves from right to left, the more recent part of its appears initiallyon the right and it remains availablefor viewing until it has reached the leftmost limit of the display. A precessing display system of this type often utilizes a circulating register. Consider a display which consists of 1,000 vertical line sweeps and which moves from right to left. In such a case, a 1,000-word register may be employed with each of the register stages containing a digital sample of the waveform to be viewed.

As each digital sample is hifted oiut oof the register, it controls the display of the dot during a respective vertical line trace, the individual dots in all of the successive line sweeps comprising the original waveform of which samples were taken. The samples are shifted out of the register at a rate which is much faster than the rate at which samples are taken. As each sample is shifted out of the register, not only does it control the display of a dot in a vertical line sweep, but it is reinserted at the input of the register. In order to update the register contents, the oldest sample is replaced by a new one after each complete recirculation of samples. Furthermore, if the number of stages of the circulating register is greater than the number of vertical lines in each frame, the effect is such that the waveform appears to move from right to left across the screen.

Typical of the signals which may be displayed in this amnner are EEG and ECG signals. Butin some cases the significance of the waveforms themselves are not readily apparent and further processing is requiredFor examkle, a blood'pressure waveform may be displayed in the manner described, but it has'been found that it is the first derivative of the blood pressure signal that contains the most important information with respect to hear muscle tonus.

In order to display the derivative of a signal together with the signal, the conventional approach is to differentiate the signal and to then use another channel of the system for viewingthesignal derivative. The overall cost of such a two-channel display approaches twice that of a one-channel display.

It is a general object of my invention to provide a precessing display of the derivative of a signal together with the signal itself at a cost which is only slightly above that of the circuitry required to display the original signal.

In accordance with the principles of my invention, successive samples are subtracted from each other as they are shifted out of the register. Since-the samples of theoriginal sample are taken at a fixed rate, the difference between successive samples is proportional to the derivative of the original signal. As each line of the display is formed, the last digital sample shifted out of the register is used to control the display of a dot as part of the original signal. The differencebetween the present sample and the previous sample is used to control the display of anothe dot in the sameline traceythis dot is part of the derivative display. Because the circulating register is required for the basic display itself, it is apparent that the additional circuitry required for the derivative display is minimal since no additional memory is necessary.

In addition to its low cost, there are several other advantages of my invention. The derivative samples are directly related to the original digital samples. Consequently, the calibration of the derivative trace is known once the calibration of the original trace is known. Furthermore, although derivatil waveforms are often verysmall in magnitude, the use of the digital sample technique allows the difference samples to be shifted prior to utilizing them to control the display of dots in the derivative waveform. Thus simply by shifting each derivative sample (which is formed by a substractor and does not require the use of a separate memory), it is possible to adjust the gain of the derivative display by successive factors of 2.

Further objects, features and advantages of the invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:

FIG. 1 depicts a digital-storage system for providing a precessing display of a signal;

FIG. 2 depicts the additional circuits required in the system of FIG. 1 to control a precessing display of the derivative of the signal together with the signal itself;

FIG. 3represents the directions of the slow and fast sweeps in the displays of FIGS. 1 and 2; and

FIGS. 4A and 48 will be helpful in understanding the manner in which the derivative display is formed.

The system of FIG. 1 includes a conventional raster display unit in which the yoke is rotated clockwise degrees relative to the yoke on the neck of a CRT in a television receiver. This causes the fast sweeps to be in the vertical direction, and the slow sweeps to be in the horizontal direction from right to left, as depicted in'FIG. 3. The vertical sweep timing is controlled by the signal at the vertical retrace input of the display.

As long as the signal is positive, a vertical sweep takes .place. But when the control signal falls in potential, the

vertical trace ceases and flyback begins. The vertical retrace input on display unit 60 is labelled to show that it is triggered on a negative edges, that is, a vertical retrace begins when the signal on conductor 54 goes low. The next vertical trace begins, following the retrace, independent of the potential on conductor 54; the width of the negative pulse does not affect the vertical sweep timing. It is only the negative step which triggers a vertical retracefollowing which the next vertical trace begins at a time determined by the time constant of the fast sweep circuit.

Similarly, the horizontal retrace is triggered by a negative step on conductor 42. Following the last vertical trace on the left side of the screen, the horizontal sweep circuit restores, but anotherhorizontal sweep does not begin automatically as a function of only the horizontal sweep circuit. Instead, the display unit includes a counter for counting negative-edge triggers .at the vertical retrace input. It is only after 100 negative retrace triggers on conductor 54 have been counted that thenext horizontal trace begins. (The use of a counter to control the period between successive horizontal sweeps in accordance with the vertical sweep triggers is a conventional technique.)

Clock '38 generates two square waveforms on conductors 40 and 42 as shown in FIG. 1. The pulses on conductor 40 occur at a rate of 100 kHz and the pulses on conductor 42 occur at a rate of 100 Hz. Conductor 40 is coupled to the trigger input of one-shot multivibrator 46. The one-shot multivibrator is triggered on the positive edge of a pulse. The output of the multivibrator is oridinarily high but it goes low when it is triggered. The output remains low for a time period determined by the setting of potentiometer 48. The width of the pulse has no effect on the vertical trace itself as described above since each vertical retrace is triggered by a negative step on conductor 54 and the next trace begins automatically following the flyback. Since the pulses on conductor 54 occur at a 100-kHz rate, microseconds are allotted for each vertical trace and retrace. Depending on the characteristics of the display unit, the trace might comprise 8 microseconds of each lO-micro-second cycle.

Since the ratio of the pulse rates on conductors 40 and 42 is l,000:l, it is apparent that there are 1,000 vertical traces for each horizontal trace. It will be recalled, however, that following a horizontal retrace trigger, another horizontal trace does not begin until after 100 vertical retrace triggers have been counted (to allow display flyback). Therefore, the display itself consists of 900 vertical sweeps per frame. During the horizontal retrace, and after the rectrace until a total of 100 vertical retrace triggers have been counted following the horizontal retrace trigger, the display unit is self-blanking. Furthermore, during each vertical retrace, the Z axis of the CRT is disabled. The only time that a display is formed is when the Z input of the display is pulsed. This occurs only momentarily during each vertical sweep and consequently only a single dot is formed on the face of the CRT during each of 900 vertical sweeps in a frame.

The 8-bit up counter 50 is provided with an 8- terminal input to which the 8 conductors in cable 30 are connected. As will be described below, the eight bits furnished to the counter represent the magnitude of a sample of the analog signal to be displayed. Each positive step on conductor 40 in addition to triggering one-shot multivibrator 46, also enables the set input of counter 50. At this time, the 8-bit sample word on cable 30 is stored in the counter. The counter is also provided with a count input connected to the output of gate 52. Each positive pulse transmitted through gate 52 increments the count initially stored in the counter. The maximum count of the counter is 255. The maximum value represented by each 8-bit sample is 255. After the initial count has been incremented to 255, the next clock pulse causes a carry pulse to be generated on conductor 58. This pulse is extended through OR gate 62 to the Z input of the display, and it is at this time that a dot is formed in the vertical trace in progress. The second input to gate 62 is labelled B and is required only if the derivative signal is to be displayed as will be described in connection with FIG. 2. If the derivative signal is not to be displayed, the carry output of the counter can be extended directly to the Z input of the display.

It is apparent that the larger the magnitude of the sample, the fewer the pulses from gate 52 which are required until a carry pulse is generated by the counter. Since each vertical sweep is in the downward direction, large-magnitude samples result in the formation of a dot in the upper portion of the display; the smallest possible sample (0) requires 256 pulses from gate 52 before a carry pulse is generated and the resulting dot which is formed is at the bottom of the display. further The count pulses are generated by display clock 56. The clock rate is determined by the setting of potentiometer 64. Since each vertical trace and retrace requires 10 microseconds, and assuming that the trace requires 8 microseconds of the 10, and further recalling that the counter must be incremented by up to 256 counts in the case of a zero sample, it is apparent that clock pulses should occur at intervals less than 8/256 microseconds.

The output of the one-shot multivibrator 46 is connected to one input of gate 52, and the output of the display clock is connected to the other. Thus clock pulses are extended through the gate to the count input of the counter only when the output of the one-shot multivibrator is high. The reason for providing potentiometers 48 and 64 is that together they control both the position of the waveform on the display and the dis- 8 play scale factor.

The positive edge of each pulse on conductor 40 triggers the multivibrator and initiates a vertical retrace. it is at this time also that gate 52 is disabled. The minimum width of the pulse at the output of multivibrator 46 approximates the vertical retrace time (for example, 2 microseconds). The next vertical trace begins automatically, even if the output of the multivibrator is still low. But as the next vertical trace begins, the counter will not start counting if the output of the multivibrator is sill low. It is onlyafter the mulvitibrator pulse terminates that display clock pulses are transmitted through gate 52. It is thus apparent that the setting of potentiometer 48 determines the level on the display of the maximum amplitude of the displayed waveform.

The scale factor is determined solely by the rate of clock 56. The initial count in counter 50 can vary between zero and 255. Thus anywhere between 1 and 256 counts are required after gate 52 is enabled before a carry pulse is generated on conductor 58. For any given sample value, the time required for the respective number of counts is determined by the rate of the clock pulses, which rate can be varied relative to the 8 microseconds allotted for each vertical trace. Consequently, the setting of potentiometer 64 determines the scale factor of the display. Typically, potentiometer 64 is adjusted so that a zero sample (which requires 256 counts) results in the display ofa dot toward the bottom of the screen.

Shift register 16 contains 1,000 8-bit parallel storage stages. Each negative step in the -kHz waveform on conductor 40 causes the contents of the shift register to be shifted from right to left. An 8-bit word thus appears at the eight terminals 44-1. Switch 44 (which, in the case of 8-bit words, represents eight individual switches) connects terminals 44-1 to terminals 44-3 in the position shown. Terminals 44-3 are connected to the eight inputs of the shift register and consequently when switch 44 is ingthe position shown the shift register functions as a circulating register. Each shift pulse causes a word to beshifted out of the left end of the register and to be re-inserted in the right end of the register. The eight bits in each sample shifted out of the register also appear on respective conductors 22-0 through 22-7.

Without considering the effect of gain control circuit 18, assume that the eight conductors 22-0 through 22-7 are connected to respective ones of the eight conductors in cable 30. The set input of the counter is enabled on each positive step in the IOOkI-lz waveform, and the contents of the shift register are rotated on each negative step in the waveform. Each negative step causes a new 8-bit sample to appear on the cable 30 and the sample is loaded in the counter immediately thereafter. Neglecting for the moment the manner in which 1,000 8-bit samples are initially stored in the shift register, it is apparent that a new sample is loaded, in the counter every 10 microseconds. The counter is loaded just as the vertical retrace begins, and the sample which is placed in the counter results in the formation of a dot sometime during the next'vertical trace. There is one sample stored in the register for each one of the 1,000 vertical traces in each display frame. As described above, since during 100 of the vertical traces the display is blanked, the actual waveform which is displayed consists of 900 dots spaced equally in the horizontal direction of the display. The dots are so close to each other, however, that the display appears to be continuous. It should be noted that the display is stationary because there are 1,000 sweeps in each frame and 1,000 stages in the shift register; the same shift register sample causes the formation of a dot in the same numbered vertical trace in every frame (with l00 of the samples not being used).

If switch 44 is moved from terminals 44-1 to terminals 44-2, a precessing displayis derived. One-word register 14 includes an 8-bit input A which is coupled to the output of shift register 16; this input is enabled when the load A (LA) input has a negative step applied to it. Thus the same negative step which controls a shift of the contents of the shift register also controls the loading of register 14 by the 8-bit word shifted out of the register. (For the moment, it is convenient to ignore the loading of register 14 by a sample applied at input B.) Each 8-bit word stored in register 14 is available via switch 44 at the input of register 16 so that the sample is re-stored in the register whenever a shift operation takes place. In effect, register 14 functions as one more stage in the overall shift register. Each sample stored in counter 50 is derived from the output of register 14 rather than the output of shift register 16.

There are thus 1,001 circulating samples while there are only 1,000 vertical traces in each frame of the display. If the vertical traces are numbered successively from right to left and a particular sample is used in one frame to control the formation of a dot in vertical sweep N, then it is apparent that the same sample will be used to control the formation of a dot in the line sweep N+1 during the next frame. In effect, each sample moves from right to left across the display, and it moves from one line trace to adjacent line trace in the time taken for a complete frame to be displayed. Since each frame requires .01 seconds and there are 900 vertical sweeps during each frame which result in visible dots, each point on the waveform requires (900) (.01) or 9 seconds to precess from right to left across the display. Thus when switch 44 is connected to terminals 44-] a stationary display can be seen, while when the switch is connected to terminals 44-2 the display moves from right to left on the screen and each point in the display, after it first appears, remains to be seen (as it moves) for nine seconds.

It is when switch 44is connected to terminals 44-2 that new samples may be stored in the digital memory. Each negative step in the waveform on conductor 42 6 triggers a horizontal retrace. Each positive step triggers the control input of analog-to-digital converter 12. Each positive step thus results in the conversion of the instantaneous magnitude of the analog signal at terminal 10 to an 8-bit digital sample which appears on the eight input leads'coupled to input B of register 14. It is the negative step in the -Hz waveform that enables the load B (L'B) input of register 14 so that the sample at the output of'the converter can be loaded in the register. It should be noted that each negative step in the 100-kHz waveform causes a sample to be shifted out of register 16 and loaded in register 14. At .Ol-second intervals, immediately after a negative step in the pulse waveform on conductor 40 has caused an 8-bit sample to be stored in register 14, the negative step on conductor 42 causes the sample to be replaced by a new sample. The sample with which is thus overwritten in register 14 is always the oldest in the digital memory. The new sample enters shift register 16 with the generation of the next shift pulse. Because the overall register contains 1,001 stages, and 1,000 shift pulses are generated for each positive step on conductor 42, it is apparent that by the time the next new sample over-writes a sample in register 14, every old sample will be displaced by one register stage from the stage which contained it when the last new sample was taken. If one looks at the overall register at a 100-Hz rate, each sample appears to precess from left to right in register 16. This means that the present new sample which is inserted in register 14 will appear in the output stage of register 16 when the next new sample is inserted in register 14, and it will appear in the next-to-last stage of register 16 when the next new sample is inserted in register 14, etc. Thus were all 1 ,000 vertical sweeps in each frame to result in the formation of a visible dot, since a new frame (horizontal retrace) begins at the same time that a new sample is loaded into register 14, the new sample would control the formation of a dot in the first vertical trace. In the next frame, since this sample will have precessed in the clockwise direction of registers 14 and 16, the sample would control the formation of a dot in the second line trace. In the next frame, the sample would control the formation of a dot in the third vertical trace, etc. Actually, because 100 vertical traces are blanked at the start of each horizontal retrace, each new sample is not seen until (100) (10 microseconds) or 1 second has elapsed subsequent to its taking.

With switch 44 connected to terminals 44-2, the digital memory is continuously up-dated so that its samples represent the last ten seconds of the analog signal. If at any time switch 44 is moved back to terminals 44-1, the display becomes stationary and no new information is inserted into the digital memory. As soon as the switch is moved to termianls 44-2, the waveform not only precesses from right to left across the screen, but the present signal (with a Z-second delay) appears on-the right side of the screen.

Terminals 44-3, in addition to being coupled back to the input of shift register 16, are connected to eight terminals identified by the letter A. The circuit of FIG. 2 has eight inputs, derived from the eight terminals represented by the letter A, as will be described below. Terminals 44-3 are also connected to the eight input conductors of gain control circuit 18. The gain control circuit is used to vary the gain of the system. The eight input conductors 22-0 through 22-7 terminate at eight respective wiper contacts 24-0 through 24-7. The eight wiper contacts are mounted on a slide 20 which also contains three grounded wiper contacts 24'. The output of the switch contains eight conductors 28-0 through 28-7 each of which is connected to a stationary one of contacts 26-0 through 26-7. In the normal position, wiper contacts 24-0 through 24-7 engage respective stationary contacts 26-0 through 26-7 and each 8-bit word at terminals 44-3 is extended through the gain control circuit to the 8-bit input of counter 50.

In the case of a low-magnitude input signal, the most significant bit of each 8-bit sample is zero. This means that the maximum count which can be loaded in counter 50 is 127 and the waveform will appear in the lower half of the display. To increase the size of the display, slider 20 is moved upward so that wiper contact 24-0 no longer engages stationary contact 26-0, and instead each of wiper contacts 24-1 through 27-7 engages a respective one of stationary contacts 26-0 through 26-6 and the uppermost one of grounded wiper contacts 24 engages stationary contact 26-7. The net effect of moving the slider upward one position is that the most significant bit in a small 8-bit sample is not used, the least significant bit in the 8-bit word loaded in the counter is always a zero, and the seven least significant bits of each 8-bit sample are coupled to the seven most significant bit inputs of the counter. This has the effect of loading the counter with a count which is twice that represented by the actual sample. In this manner, the displayed waveform is doubled in size. Similarly, by moving the slider up one more position the resulting gain is 4. With three terminals 24' as shown in FIG. 1, a maximum gain of 8-is possible. Thus once potentiometers 48 and 64 are adjusted and the system is calibrated, the gain can be increased by successive factors of 2 without any additional calibrations being necessary.

In order to display the derivative of the waveform, the 8-bit sample words at terminals 44-3 are applied to the C input of 8-bit parallel subtractor 72 in FIG. 2 and the input of l-word register 70. The 8-bit output of register 70 is coupled to the D input of the subtractor. Conductor 40 is connected to the load input of register 70. A new 8-bit sample is stored in register 70 whenever a positive step appears on conductor 40. Since it is a positive step on conductor 40 which shifts the contents of registers 14 and 16 in FIG. 1, it is apparent that the 8-bit sample at input I) of the subtractor is always the sample just behind that an input C of the subtractor. I

The output of the subtractor is an 8-bit difference sample which is extended through gain control circuit 74 to the 8 input conductors coupled to S-bit up counter 80. The gain control circuit 74 is the same as gain control circuit 18 in FIG. 1 and functions to increase the size of the derivative waveform. The most significant bit in the difference sample (after amplification) appears on conductor MSB and the least significant bit appears on conductor LSB. Reference numeral 78 represents an inverter, as indicated by the letter I. The most significant bit is inverted by inverter 78 for a reason to be described below. I

Counter 80 operates as does counter 50 in FIG. 1. One-shot multivibrator 82 is triggered by each positive step on conductor 40 and the width of its output pulse is determined by the setting of potentiometer 84. Only when the output of the multivibrator is high in potential is gate 90 enabled, and only at this time are clock pulses from display clock 86 extended through the gate to the count input of counter 80. The rate of the clock pulses is controlled by the setting of potentiometer 88. At the same time that a new sample is loaded into counter 80, the most recent difference sample is loaded into counter (with the triggering of the set input of the counter). The clock pulses applied to the count input of the counter eventually result in a carry pulse being generated by the counter. The carry pulse, which appears at terminal B, is extended through OR gate 62 in FIG. 1 to the Z input of the display. Consequently, during each vertical trace one dot is formed when a carry pulse is generated by counter 50 for the original waveform, and another dot is formed'with the generation of a carry lpulse by counter 80 for the derivative of the original waveform; two dots are formed during each vertical trace. Multivibrator 82 and clock 86 serve the same functions as mulvibrator 46 and clock 56; they control the position and scale of the derivative display. It should be noted that if the rates of clocks 56 and 86 are the same, then the two waveforms which are seen will have the same-scale factors (except for respective gain factors which are multiples of 2 and which are determined by the two gain control circuits). This means that if the system is calibrated for the original waveform, no additional calibration is required for the display of the derivative waveform. Furthermore, if the two one-shot multivibrators generate pulses of the same width (i.e., the same one of gates 52 and 90 is coupled to the count inputs of both counters), then both waveforms will have the same base line on the disp y- It is important to note that the additional circuitry depicted in FIG. 2 does not include a digital memory. The data stored in the registers of FIG. 1 is also used to form the derivative waveform. Difference samples are not even stored. Instead, a new derivative sample is derived prior to the start of each trace simply by subtracting the next-to-the-last sample shifted out of register 16 from the present sample. Very little circuitry is required to provide the additional derivative waveform display. It is sufficient to determine the values of successive differences because samples of the original waveform are taken at a constant rate. (Since in the limiting case the derivative of a signal is equal to the difference between two values of a signal divided by the time interval between them, and samples of the signal at terminal 10 in FIG. 1 are taken at a IOO-I-Iz rate, the actual value of the derivative signal displayed is times the magnitude observed on the display.)

The reason for inverter 78 is the following. Subtractor 72 forms the difference between the samples at inputs C and D. Assume that the sample at input C is slightly larger than the sample at input D. In such a case there results a difference sample whose most significant bits are Os. A small difference sample is loaded into counter 80 and this results in a dot being formed at the bottom of the display. FIG. 4A depicts the manner in which a sine wave'would be displayed. The positive portion of the signalwould appear at the bottom of the display.

On the other hand, if the sample at input D of the subtractor is slightly larger than the sample at input C, then the subtractor furnishes the 2s complement of the negative result. (The borrow bit is lost.) In such a case, the most significant bits of the difference sample are ls. When a high-value sample is loaded into counter 80, there results the formation of a dot at the top of the display. Thus the negative part of the sine it is inverted, the most significant bit set in counter 80 is a 1. This has the effect of raising each dot which is formed by one-half of the vertical height of the display. Thus, the positive portion of the sine wave being con sidered appears above the dotted base line as shown in FIG. 48. With respect to a sample at input D which is greater than a sample at input C, the most significant bit is a 1. When this bit is inverted, the most significant bit loaded into counter 80 is a 0. This has the effect of lowering the dot which is formed by one-half of the height of the display. This is shown in FIG. 4B with the negative portion of the sine wave being below the dotted base line. By inverting the most significant bit of each difference sample in this manner, the negative and positive portions of the derivative waveform are properly joined to each other.

Although the waveform in FIG. 4B is shown as being centered in the middle of the display, the actual position of the derivative waveform depends, of course, on the settings of potentiometers 84 and 88. The four potentiometers can be set so that the two displayed waveforms overlap each other or are respectively positioned in the upper and lower halves of the screen. The inversion of the most significant bit of each difference sample does not preclude the use of gain control circuit 74 provided that the gain control is not set so high that the most significant bit at the input of inverter 78 has a value different from the most significant bit at the output of the subtractor.

It should be noted that if switch 44 is connected to terminals 44-1 rather than terminals 44-2, then the derivative waveform display is stationary. Whether the original signal is continuously up-dated and precessed across the screen or it remains stationary, the derivative waveform always appears with it along the same time base. This is because one of the two samples which is used to generate each dot for the derivative waveform is always the sample which'is being used to generate a dot in the display of the original waveform.

Although the invention hasv been described with reference to a particular embodiment, it is to be understood that this embodiment is merely'illustrative of the application of the principles of the invention. For example, it is possible to convert two successive digital samples to analog signals and to use an operational amplifier to derive the analog difference between them; the analog difference signal could be used together with a comparator and a ramp generator which is synchronous with the vertical sweep to generate the'derivative waveform dots. Also, even if the original waveform is not displayed, the derivative waveform can still be displayed. (The samples in the shift register might be used for some purpose other than to display the waveform.) Thus it is to be understood that numerous modi-' fications may be made in theillustrative embodiment of the invention and other arrangements may be devised without departing from the spirit and scope of the invention.

What I claim is:

1. A system for displaying a continuous signal waveform and its derivative waveform comprising means for stroing successive digital samples of said digital waveform, means for reading out from said storing means time-successive digital samples of said signal waveform, first means for forming a raster display and for controlling the formation of a'dot at a point along each vertical sweep of the display in manner that creates a continuous visual effect and whose position is dependent upon the magnitude of a respective digital sample read out from said storing means, means for deriving the difference between time-successive digital samples read out from said storing means, second means for controlling the formation of another dot at a point along each vertical sweep of said display whose position is dependent upon the magnitude of said difference, and means for inverting the most significant bit of each difference digital value prior to an operation thereupon by said second controlling means.

2. A system in accordance with claim 1 wherein digital samples are read out from said storing means during successive vertical sweep cycles of said display means, and each digital sample is read out from said storing means a number of times and at a rate which is slower than the horizontal sweep cycle of said display means so that the signal waveform and the derivative waveform which are displayed on said display means precess thereacross.

3. A system inj accordance withclaim 2 further including means for changing the rate at which each digital sample is read out from said storing means to equal the horizontal sweep rate of said display means so that the signal waveform and the derivative waveform which are displayed on said display means remain stationary.

4. A system in accordance with claim 2 wherein said storing means includes shift register means having a number of stages greater than the number of vertical sweep cycles which take place during each horizontal sweep cycle, each of said stages containing a digital sample therein with the digital samples shifted out of said shift register means being used to form the dots along the vertical sweeps of said display.

5. A system in accordance with claim 4 further in cluding measnfor converting the instantaneous magnitude of said signal waveform to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.

6. A system in accordance with claim 5 wherein said first and second means for controlling'the formation of dots at points along a vertical sweep each includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of a digital sample or the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said first or second controlling means thereupon fd'r changing the scale factor of a displayed waveform by a factor of two.

7. A system in accordance with claim 1 wherein said storing means includes shift register means having a number of stages greater than the number of vertical sweep cycles which take place during each horizontal sweep cycle, each of said stages containing a digital sample therein with the digital samples shifted out of said shift register means being used to form dots along the vertical sweeps of said display.

8. A system in accordance with claim 7 further including means for converting the instantaneous magnitude of said signal waveform to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.

9. A system in accordance with claim 7 wherein said first and second means for controlling the formation of dots at points along a vertical sweep each includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of a digital sample or the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said first or second controlling means thereupon for changing the scale factor of a displayed waveform by a factor of two.

10. A system in accordance with claim 1 further including means for converting the instantaneous magnitude of said signal waveform to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.

11. A system in accordance with claim 1 wherein said first and second means for controlling the formation of dots at points along a vertical sweep each includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of a digital sample or the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said first or second controlling means thereupon for changing the scale factor of a displayed waveform by a factor of two.

12. A system in accordance with claim 1 wherein said deriving means derives the digital value of each difference.

13. A system for displaying the derivative waveform of a continuous signal comprising means for storing successive digital samples of said signal, means for reading out from said storing means at a fixed rate time-successive digital samples of said signal, means for deriving the difference between time-successive digital samples read out from said storing means, means for forming a raster display and for controlling the formation of a dot at a point along each vertical sweep of said display in a manner'that creates a continuous visual effect whose position is dependent upon the magnitude of a respective difference, and means for inverting the most significant bit of each digital value prior to an operation thereupon by said controlling means.

14. A system in accordance with claim 13 wherein digital samples are read out from said storing means during successive vertical sweep cycles of said display means, and each digital sample is read out from said storing means a number of times and at a rate which is slower than the horizontal sweep cycle of said display means so that the derivative waveform which is displayed on said display means precesses thereacross.

15. A system in accordance with claim 14 further including means for changing the rate at which each digital sample is read out from said storing means to equal the horizontal sweep rate of said display means so that the derivative waveform which is displayed on said display means remains stationary.

16. A system in accordance with claim 13 wherein said storing means includes shift register means having a number of stages greater than the number of vertical sweep cycles which take place during each horizontal sweep cycle, each of said stages containing a digital sample therein with successive digital samples shifted out of said shift register means being used to form successive differences operated upon by said controlling means.

17. A system in accordance with claim 16 further including means for converting the instantaneous magnitude of a signal to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.

18. A system in accordance with claim 13 further including means'for converting the instantaneous magntiude of a signal to a digital sample, and means for operating periodically said converting means and for replacing an old-digital sample in said storing means by the most recent digital sample.

19. A system in accordance with claim 13 wherein said means for controlling the formation of a dot at a point along a vertical sweep includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said controlling means thereupon for changing the scale factor of the displayed waveform by a factor of two.

20. A system in accordance with claim 13 wherein said deriving means derives the digital value of each difference. 

1. A system for displaying a continuous signal waveform and its derivative waveform comprising means for storing successive digital samples of said digital waveform, means for reading out from said storing means time-successive digital samples of said signal waveform, first means for forming a raster display and for controlling the formation of a dot at a point along each vertical sweep of the display in a manner that creates a continuous visual effect and whose position is dependent upon the magnitude of a respective digital sample read out from said storing means, means for deriving the difference between time-successive digital samples read out from said storing means, second means for controlling the formation of another dot at a point along each vertical sweep of said display whose position is dependent upon the magnitude of said difference, and means for inverting the most significant bit of each difference digital value prior to an operation thereupon by said second controlling means.
 2. A system in accordance with claim 1 wherein digital samples are read out from said storing means during successive vertical sweep cycles of said display means, and each digital sample is read out from said storing means a number of times and at a rate which is slower than the horizontal sweep cycle of said display means so that the signal waveform and the derivative waveform which are displayed on said display means precess thereacross.
 3. A system in accordance with claim 2 further including means for changing the rate at which each digital sample is read out from said storing means to equal the horizontal sweep rate of said display means so that the signal waveform and the derivative waveform which are Displayed on said display means remain stationary.
 4. A system in accordance with claim 2 wherein said storing means includes shift register means having a number of stages greater than the number of vertical sweep cycles which take place during each horizontal sweep cycle, each of said stages containing a digital sample therein with the digital samples shifted out of said shift register means being used to form the dots along the vertical sweeps of said display.
 5. A system in accordance with claim 4 further including means for converting the instantaneous magnitude of said signal waveform to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.
 6. A system in accordance with claim 5 wherein said first and second means for controlling the formation of dots at points along a vertical sweep each includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of a digital sample or the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said first or second controlling means thereupon for changing the scale factor of a displayed waveform by a factor of two.
 7. A system in accordance with claim 1 wherein said storing means includes shift register means having a number of stages greater than the number of vertical sweep cycles which take place during each horizontal sweep cycle, each of said stages containing a digital sample therein with the digital samples shifted out of said shift register means being used to form dots along the vertical sweeps of said display.
 8. A system in accordance with claim 7 further including means for converting the instantaneous magnitude of said signal waveform to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.
 9. A system in accordance with claim 7 wherein said first and second means for controlling the formation of dots at points along a vertical sweep each includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of a digital sample or the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said first or second controlling means thereupon for changing the scale factor of a displayed waveform by a factor of two.
 10. A system in accordance with claim 1 further including means for converting the instantaneous magnitude of said signal waveform to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.
 11. A system in accordance with claim 1 wherein said first and second means for controlling the formation of dots at points along a vertical sweep each includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of a digital sample or the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said first or second controlling means thereupon for changing the scale factor of a displayed waveform by a factor of two.
 12. A system in accordance with claim 1 wherein said deriving means derives the digital value of each difference.
 13. A system for displaying the derivative waveform of a continuous signal comprising means for storing successive digital samples of said signal, means for reading out from said storing means at a fixed rate time-successive digital samples of said signal, means for deriving the difference between time-successive digital samples read oUt from said storing means, means for forming a raster display and for controlling the formation of a dot at a point along each vertical sweep of said display in a manner that creates a continuous visual effect whose position is dependent upon the magnitude of a respective difference, and means for inverting the most significant bit of each digital value prior to an operation thereupon by said controlling means.
 14. A system in accordance with claim 13 wherein digital samples are read out from said storing means during successive vertical sweep cycles of said display means, and each digital sample is read out from said storing means a number of times and at a rate which is slower than the horizontal sweep cycle of said display means so that the derivative waveform which is displayed on said display means precesses thereacross.
 15. A system in accordance with claim 14 further including means for changing the rate at which each digital sample is read out from said storing means to equal the horizontal sweep rate of said display means so that the derivative waveform which is displayed on said display means remains stationary.
 16. A system in accordance with claim 13 wherein said storing means includes shift register means having a number of stages greater than the number of vertical sweep cycles which take place during each horizontal sweep cycle, each of said stages containing a digital sample therein with successive digital samples shifted out of said shift register means being used to form successive differences operated upon by said controlling means.
 17. A system in accordance with claim 16 further including means for converting the instantaneous magnitude of a signal to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.
 18. A system in accordance with claim 13 further including means for converting the instantaneous magnitude of a signal to a digital sample, and means for operating periodically said converting means and for replacing an old digital sample in said storing means by the most recent digital sample.
 19. A system in accordance with claim 13 wherein said means for controlling the formation of a dot at a point along a vertical sweep includes means for forming a dot in said raster display a distance measured away from a horizontal reference line which is proportional to the digital value of the difference between successive digital samples, and further including means for shifting a digital value prior to the operation of said controlling means thereupon for changing the scale factor of the displayed waveform by a factor of two.
 20. A system in accordance with claim 13 wherein said deriving means derives the digital value of each difference. 